1. Field of the Invention
The invention relates to a method for fabricating a CMOS image sensor.
2. Description of the Prior Art
As the development of electronic products progresses, the demand for related components has increased as well. For example, as the development of digital cameras and scanners progresses, the demand for image sensor increases accordingly. In general, today's image sensors in common usage are divided into two main categories: charge coupled device (CCD) sensors and CMO image sensors (CIS). The application of CMOS image sensors has increased significantly for several reasons. Primarily, CMOS image sensors have certain advantages of offering low operating voltage, low power consumption, and the ability for random access. Additionally, CMOS image sensors are currently capable of integration with the semiconductor fabrication process.
The CMOS image sensor separates (i.e., classifies) incident light into a combination of light of different wavelengths. The light of different wavelengths is received by respective sensing elements and is subsequently transferred into digital signals of different intensities. For example, the CMOS image sensor can consider incident light as a combination of red, blue, and green light. Those wavelengths are subsequently received by photodiodes, and then transformed into digital signals. However, in order to separate incident light, a monochromatic color filter array (CFA) must be set above every optical sensor element.
Please refer to FIGS. 1-4. FIGS. 1-4 illustrate a method for fabricating a CMOS image sensor according to the prior art. As shown in FIG. 1, a semiconductor substrate 12 having a sensor array region 14 and a peripheral region 16 is provided. A plurality of photodiodes 18, CMOS transistors (not shown), and shallow trench isolations 20 surrounding the photodiodes 18 are formed on the semiconductor substrate 12, and a dielectric layer 22 is deposited on the semiconductor substrate 12 thereafter. Next, a plurality of patterned metal layers 24 is disposed on the dielectric layer 22 of the sensor array region 14 for forming metal interconnects of the CMOS image sensor and a patterned metal layer is disposed on the dielectric layer 22 of the peripheral region 16 for serving as a contact pad 26. Another dielectric layer 28 is disposed on the dielectric layer 22 covering the metal layer 24 and the contact pad 26 thereafter. Next, a patterned photoresist (not shown) is disposed on the dielectric layer 28 and an etching process is performed by using the patterned photoresist as a mask to remove a portion of the dielectric layer 28 for forming an opening 30 in the dielectric layer 28 and exposing the surface of the contact pad 26.
As shown in FIG. 2, a cap layer 32 is disposed on the dielectric layer 28 and the contact pad 26, in which the cap layer 32 is composed of titanium or titanium nitride. As shown in FIG. 3, a patterned photoresist (not shown) is disposed on the cap layer 32 and an etching process is performed by using the patterned photoresist as a mask to remove a portion of the cap layer 32 for forming an optical shielding layer 34 on the dielectric layer 28 of the peripheral region 16 adjacent to the sensor array region 14. The optical shielding layer 34 preferably serves to block external lights projecting from the peripheral region 16 to the sensor array region 14.
As shown in FIG. 4, a plurality of color filtering layers (not shown) is disposed on the dielectric layer 28 of the sensor array region 14, and a series of exposure and development processes are conducted to form a plurality of color filters 36 corresponding to each of the photodiodes 18 disposed in the semiconductor substrate 12. The color filters 36 are composed of red color filters, green color filters, blue color filters or filters of other colors. Thereafter, a planarizing layer 38 is formed on the color filters 36 and the optical shielding layer 34, and a polymer layer (not shown) composed acrylate material is disposed on the planarizing layer 38. Next, a series of exposure, development, and baking process are performed to form a plurality of microlenses 40 corresponding to the color filters 36, thus complete the fabrication of a CMOS image sensor 42.
It should be noted that the a series of exposure and development processes are usually conducted to fabricating optical devices including color filters, planarizing layer, and microlenses. Since the contact pad disposed on the peripheral region of the CMOS image sensor is typically composed of aluminum, and the surface of the contact pad is exposed during the fabrication process, the developer used for the development process will often corrode the exposed contact pad and result in a pitting phenomenon. This phenomenon not only produces a plurality of recess on the surface of the pad, but also influences the reliability of the pad for conducting wire bonding in the later process.